Car overview and mechanics: the big parts
A car has four big groups: the engine (makes power), the power train (carries power to the wheels), the body and chassis (holds everything, steers and cushions) and the brakes (stop it).
Two forces decide how a car moves. The driving force from the tyres pushing back on the road moves the car forward. The resistance (air drag, tyre rolling friction, and slope if uphill) tries to slow it. If driving force > resistance, speed rises. If equal, speed is steady. If less, speed falls.
Air drag grows fast with speed: double the speed, about four times the drag. That is why high speed uses much more fuel.
The four-stroke engine
An engine turns the heat of burning fuel into motion. Inside a cylinder a piston slides up and down. One full cycle has four strokes:
- Intake: piston goes down, the intake valve opens, air and fuel enter.
- Compression: both valves shut, piston goes up and squeezes the mixture.
- Power: a spark lights the mixture. Hot gas pushes the piston down hard. This is the only stroke that gives power.
- Exhaust: the exhaust valve opens, piston goes up and pushes the burnt gas out.
The piston turns a crankshaft, which spins. Real engines have 3, 4 or more cylinders firing one after another, so the shaft turns smoothly. A diesel engine works the same way but squeezes only air so hard that it becomes hot enough to light the fuel without a spark.
Power-train devices: clutch, gearbox, shaft, differential
- Clutch: joins or cuts the engine from the gearbox, so the car can stand still with the engine running and change gear smoothly.
- Gearbox (transmission): gear wheels of different sizes. Low gear: big push, low speed (start, hills). High gear: small push, high speed (cruising). Neutral: no link.
- Drive shaft: a spinning rod carrying power to the axle.
- Differential: splits power between the left and right wheels and lets them turn at different speeds, because the outer wheel travels farther on a bend.
- Final drive and axle: one last gear reduction, then the axle turns the wheels.
Gear ratio = engine turns ÷ wheel turns. Ratio 3.6 means the engine turns 3.6 times for one wheel turn: speed ÷ 3.6, push × 3.6. Front-wheel, rear-wheel and four-wheel drive differ in which wheels receive the power. An electric car replaces the engine with a motor and usually needs only one gear.
Operating the car
The driver uses three main controls. The accelerator (throttle) decides how much air and fuel enter, so how much power. The clutch and gear lever pick the gear (an automatic does this itself). The steering wheel turns the front wheels to change direction. The dashboard shows speed, engine speed (rpm), fuel and warning lights.
Start: press the clutch, select first gear, release the clutch slowly while pressing the accelerator. Go up through the gears as speed rises, and drop to a lower gear for hills or overtaking.
Braking: how a car stops
Pressing the pedal pushes brake fluid through pipes. The fluid squeezes pads against a spinning disc (or shoes against a drum). Friction slows the wheel, and the motion energy turns into heat. Fluid carries force equally everywhere, so one foot can stop four wheels. Brakes are in pairs so the car does not pull to one side.
Stopping distance = thinking distance (travelled before your foot moves) + braking distance. Braking distance grows with the square of speed: twice the speed needs four times the distance. Wet roads and worn tyres make it longer. Hot brakes fade, so use a lower gear on long downhills.
Key formulas and definitions
- Net force = driving force − resistance (speed rises if > 0)
- Gear ratio = engine turns ÷ wheel turns
- Wheel push ≈ engine push × gear ratio; wheel speed ≈ engine speed ÷ gear ratio
- Braking distance ∝ speed² (double speed, 4 times distance)
- Key terms: piston, crankshaft, clutch, differential, throttle, friction
Worked examples
1. An engine turns 3000 times a minute in a gear of ratio 3. How many times does a wheel turn per minute?
Wheel turns = engine turns ÷ ratio = 3000 ÷ 3 = 1000 per minute.
2. The driving force is 900 N and the resistance is 600 N. The car mass is 1200 kg. Find the acceleration.
Net force = 900 − 600 = 300 N. a = F ÷ m = 300 ÷ 1200 = 0.25 m/s².
3. A car runs at steady speed. The drag is 700 N. What is the driving force?
Steady speed means net force 0, so driving force = 700 N.
4. A car needs 20 m to stop from 30 km/h. About how far from 60 km/h on the same road (braking only)?
Distance ∝ speed². Double speed gives 4 times: 20 × 4 = 80 m.
5. A wheel of radius 0.3 m turns 5 times per second. Find the car speed.
Distance per turn = 2π × 0.3 = 1.885 m. Speed = 1.885 × 5 = 9.4 m/s ≈ 34 km/h.
6. Two cylinders of a four-stroke engine are out of step so that one gives a power stroke while the other is on a different stroke. Why is that useful?
Only one stroke in four gives power. If cylinders fire one after another, there is a power push almost all the time, so the engine runs smoothly.
Common mistakes
- Saying all four strokes make power. Only the power stroke does; the other three use energy.
- Thinking high gear always means more power. High gear gives more speed but less push.
- Thinking the brake makes the energy vanish. It turns motion energy into heat.
- Forgetting that stopping distance has two parts: thinking distance and braking distance.